Command blocks on large stdin

[rocketnik]

using macOS 10.13.4, ghc 8.0.2, shelly-1.7.2

the following command blocks indefinitely:

ghci -XOverloadedStrings
:m +Data.List Data.Text Shelly
shelly $ setStdin (pack $ Prelude.take 1000000 . repeat $ 'a') >> (fmap (Data.Text.take 200) $ run "cat" []) >>= echo

if you replace 1000000 with something shorter like 100000 or 1000, then this command succeeds. Looks like a buffering/laziness issue.